1. Field of the Invention
The present invention relates generally to the field of audio delay equalization in simulcast transmission systems, and more particularly to a method and apparatus for transmission path delay measurements using adaptive demodulation.
2. Description of the Prior Art
Simulcast transmission systems are widely used to distribute information, such as voice messages and numeric or alphanumeric data messages within paging systems, or data messages within data communication systems, such as in information service systems. In order for simulcast transmission systems to reliably distribute information throughout the system, precise control and optimization of transmission path delays between the originating equipment and the receiving equipment is required. In many prior art simulcast transmissions systems, this has previously been achieved by equipping all fixed base station equipment with analog or digital delay lines capable of equalizing the different control path delays between the base station equipment and control station equipment. In some of the prior art simulcast systems, the delay lines have been manually set based on air mileage calculations and rough delay estimates, and as a result, typically have provided limited delay equalization due to the reliance on poor phase delay estimates. Many simulcast transmission systems have also relied on a volatile control path medium, such as telephone lines for distribution of information between the base station equipment and control station equipment. As a consequence, a number of methods for providing automatic phase delay equalization have been proposed and implemented to regularly equalize ever changing transmission path delays. When automatic equalization methods are utilized, however, the delay measurement apparatus must be tolerant of the common telephone and RF path impairments which are encountered, such as random noise, group delay distortion, phase jitter, impulse response and harmonic distortion. Prior art automatic delay equalization equipment, however, has been plagued by poor performance over realistic transmission media and has typically failed to work under extreme conditions. In particular, many of the prior art methods have attempted to derive accurate timing information from an analog signal's zero crossings, while extracting coarse timing information from tone frequency transitions. Unfortunately, the correlation between the time domain and the frequency domain is inexact and subject to large inaccuracies, as much as half to full cycles of the analog tone in the final measurement due to group delay distortion on the transmission channel. Thus what is needed to provide reliable automatic transmission path delay measurements and phase delay equalization is a method and apparatus for providing extremely accurate measurements under ideal conditions, and very accurate measurements under extreme conditions. The method and apparatus must also be capable of distinguishing an extremely poor measurement from an adequate measurement, thus minimizing false readings during the transmission delay measurement process.